This invention relates generally to nuclear reactors and more particularly, to apparatus for repairing jet pump assemblies within a nuclear reactor pressure vessel.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus, provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The restrainer brackets support the inlet mixer by attaching to the adjacent jet pump riser pipe. The purpose of the gravity actuated wedge is to maintain contact between the inlet mixer and the restrainer bracket. The wedge works in cooperation with two set screws which are tack welded to the restrainer bracket to maintain contact with the inlet mixer. The flow of water through the jet pumps typically includes pressure fluctuations which are caused by various sources in the reactor system. The pressure fluctuations may have frequencies close to one or more natural vibration modes of the jet pump piping. The jet pump piping stability depends on the tight fit-up, or contact, of the restrainer brackets and the inlet mixers. Operating thermal gradients, hydraulic loads, and fluctuations in the hydraulic loads can overcome the lateral support provided by the gravity wedge, allowing gaps or clearances to develop at the opposing two fixed contacts or set screws. Particularly, the tack welds can break and the set screws can loosen permitting the jet pump to vibrate within the restrainer bracket. The loss of contact between the inlet mixer and the restrainer bracket can change the jet pump natural frequency to match some excitation frequency in the system, causing vibration of the piping and exerting increased loads which may cause cyclic fatigue cracking and wear of the piping supports, which can result in degradation from wear and fatigue at additional jet pump structural supports.
To overcome this problem, gravity wedge supports have been used at locations where clearances have developed in restrainer bracket contacts. The gravity wedge support employed a sliding wedge and a fixed bracket mount which engaged the jet pump restrainer bracket. To allow access for installation of the wedge support required disassembly of the jet pumps, which is an undesirable expense and may cause an extension of reactor maintenance downtime. Additionally, the gravity wedge supports typically included bolted attachments which could vibrate loose and fall into the reactor. Another attempted solution is to reinforce the welded attachment of the two set screws to the restrainer bracket, then reset the inlet mixer against the set screws when the jet pump is reassembled. However, this procedure causes significant downtime and also requires disassembling the jet pumps.
It would be desirable to provide an apparatus for restoring the tight rigid fit-up provided between the inlet mixer and the adjacent restrainer bracket, replacing the support function of the existing screw type contacts. It would also be desirable to provide an apparatus that would prevent the set screws from backing out completely and escaping into the reactor system. Additionally, it would be desirable to provide an apparatus that can be remotely installed by attachment to the existing restrainer bracket without disassembling the inlet mixer, and remain in place during disassembly of the jet pumps during maintenance shutdowns.